A:  First, you must write a query that uses a variable.

Select Test."FName", Test."Salary Of Employee"
From Test
Where Test."Salary of Employee" > :val

Note:  If you just write the field name as "Salary of Employee" you will get a Capability Not Supported error.  It must be Test."Salary of Employee".

In this can the variable name is "val", but it can be whatever you want (of course).  Then, you go to the TQuery's params property and set the "val" param to whatever the appropriate type is.  In our example here we will call it an integer.

Next, you write the code that sets the param's value.  We will be setting the value from a TEdit box.

procedure TForm1.Button1Click(Sender: TObject);
begin
  with Query1 do
  begin
    Close;
    ParamByName('val').AsInteger := StrToInt(Edit1.Text);
    Open;
  end;
end;
 

Note:  you may want to place this code in a try..except block as a safety precaution.

If you want to use a LIKE in your query, you can do it this way:

Select * From customer
Where company like :CompanyName

procedure TForm1.Button1Click(Sender: TObject);
begin
  with Query1 do
  begin
    Close;
    ParamByName('CompanyName').AsString := Edit1.Text + '%';
    Open;
  end;
end;

The trick is in the concatenating of the percentage sign at the end of the parameter.